bpo-42979: Enhance abstract.c assertions checking slot result (GH-24352)

bpo-42979: Enhance abstract.c assertions checking slot result (GH-24352)

* bpo-42979: Enhance abstract.c assertions checking slot result

Add _Py_CheckSlotResult() function which fails with a fatal error if
a slot function succeeded with an exception set or failed with no
exception set: write the slot name, the type name and the current
exception (if an exception is set).